The term irregular is often used to describe a combatant who belongs to a paramilitary group, militia, volunteer corps, organized resistance movement, or rebel force. Irregulars are frequently part-time combatants who do not wear a uniform or carry arms openly when on active duty. However, irregulars can also be part of a country’s armed forces, as they are in Switzerland, where the army is composed almost entirely of uniformed militias.

Irregular is not necessarily a synonym for guerrilla. Guerrillas are fighters distinguished by their use of tactics such as ambushes, sniping, and sabotage. Irregulars might not use such tactics at all, while regular armed forces often do.

In internal armed conflicts, the most important characteristic of irregulars is that they prefer to blend into the civilian population and thus, often, endanger civilians as government forces will destroy or otherwise punish entire villages or towns in an attempt to neutralize rebel irregulars.

Partisan is commonly used to describe irregulars who resist the occupation of a country by a foreign power—for example, French Maquis in World War II. Partisans might operate inside or outside occupied territory.

In international conflicts, irregulars may be considered lawful combatants, entitled to prisoner of war status if they adhere to certain standards. These include that they: distinguish themselves from the civilian population (i.e., look like combatants); carry weapons openly during engagements or deployments; be commanded by a responsible officer and, generally, be expected to comply with international rules relating to armed conflict. Failure to meet these standards can lead to trial and punishment for hostile acts. (Mercenaries, within the legal definition set forth under the 1977 Additional Protocol I, are not entitled to prisoner of war status.)

In internal armed conflict there is no prisoner of war status and the government is free to try its armed enemies for treason or other violent acts. Each trial, however, must be in a “regularly constituted court, affording all the judicial guarantees which are recognized as indispensible by civilized people” according to Article 3 common to the Geneva Conventions of 1949.